SPI link testing continues with some success; Wiring the Virtual/Real switch into the IBM 1130

DIAGNOSTIC INFORMATION I WILL WATCH

The SPI state machine moves through a number of states from the first to the 325th word being exchanged. To verify that it is reaching the end properly I emitted some signals that can be displayed on the scope alongside the end of transaction (SPItransaction going low). 

The state machine reaches a state SBcheckA when it has finished with the 321st data word and is preparing to send and receive the checksum of those data words. It then passes through SBtest where it verifies if an error free transaction has been received. Near the very end it reaches SBflagB state. 

I am also monitoring the command it believes it has received. One line goes high when it sees the bit pattern SCdrive that sets the RAM access to the drive side. Another line is the default state SCspi for when the RAM access should be connected to the SPI link. If an error condition is detected during the transaction, I would expect to see the command switch from SCspi to SCdrive for a while then revert before the end of the transaction. 

TESTING RESULTS AND OBSERVATIONS

I worked on various issues including not setting the command state to SCspi at power on reset, something that would have no impact on the system in real operation but needed repair to have a clean design. With the new instrumentation I discovered a small flaw, corrected it, and was pleased to see that the FPGA board switched the RAM over to drive mode from SPI mode - a successful transaction completion.

It appears however that something gets wedged after that one transaction, as I never latch in the subsequent command to switch back to SPI mode, while I can see the properly formatted transaction being received. I changed the diagnostic outputs and watched the state machine working well for the first transaction and it should be resetting to the idle state when the SPItransaction signal is dropped. 

Next up is a careful review of all the state machines and how they reset. It may be that I need to control how they move out of reset to avoid whatever is taking place to lock it up. I will also produce some new diagnostic output signals to help me find and resolve the issue causing this behavior. 

SWITCH BEING WIRED TO SWITCH DRIVE BETWEEN REAL AND VIRTUAL MODE

My modification to support the Virtual 2315 Cartridge Facility is a switch that reroutes signals that ran to the Head Load solenoid and the Heads Loaded microswitch. In the Real mode, signals are wired as usual, but in Virtual mode, the solenoid is out of the loop. 

For Virtual mode, the signal that would activate the solenoid is instead routed up to the Arduino inside my facility. The Heads Loaded signal line to the drive electronics is disconnected from the microswitch and instead routed to the Arduino. This lets the Arduino see that the drive electronics is requesting the heads to be down on the platter surface and the Arduino can lie to the electronics, telling it that the heads have indeed come down. 

I bought some final connectors and a mounting plate to install the switch in position on the frame of the IBM 1130 behind the disk drive. The remainder of the wiring and the physical installation will take place over the next few days.